[x86] Teach the new vector shuffle lowering the basics about insertion
of a single element into a zero vector for v4f64 and v4i64 in AVX.
Ironically, there is less to see here because xor+blend is so crazy fast
that we can't really beat that to zero the high 128-bit lane.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218214 91177308-0d34-0410-b5e6-96231b3b80d8